Closed loop control for refractive laser surgery (LASIK)

Abstract

A closed-loop control system for the superficial photoablation of stromal tissue from an exposed surface includes an active mirror for directing a diagnostic laser beam through the exposed surface to a focal point on the retina of an eye. The reflected beam is analyzed by a detector to identify a distorted wavefront that is indicative of required corrections, and an induced wavefront that is indicative of optical aberrations that have been mechanically introduced when the stromal tissue was exposed. A compensator alters a desired wavefront with the induced wavefront to create a rectified wavefront. A comparator then compares the rectified wavefront with the distorted wavefront to create an error signal. In the operation of the closed-loop control system, the error signal is used to maintain a focal point on the retina for the diagnostic laser beam. Also, at a null, the error signal indicates when the required amount of stromal tissue has been photoablated.

What is claimed is:  
     
       1. A closed-loop control system for superficial photoablation of tissue from an exposed stromal surface of an eye wherein the exposed surface introduces mechanically induced optical aberrations, said system comprising: 
       a source for generating an incising laser beam to photoablate tissue from the exposed surface;  
       a source for generating a diagnostic laser beam;  
       a deformable mirror for directing said diagnostic laser beam through the exposed stromal surface to a focal spot on the retina of the eye;  
       a detector for using light from said diagnostic beam reflected from the retina through the exposed stromal surface to generate an induced wavefront having characteristics of the mechanically induced optical aberrations, together with a distorted wavefront having actual real-time characteristics of the cornea;  
       a compensator for altering a predetermined desired wavefront by incorporating said induced wavefront therewith to create a rectified wavefront;  
       a comparator for comparing said rectified wavefront with said distorted wavefront to create an error signal;  
       a means for reconfiguring said deformable mirror in accordance with said error signal to maintain said focal spot on the retina; and  
       a means for ceasing generation of said incising laser beam when said error signal is a null.  
     
     
       2. A system as recited in  claim 1  wherein said source for generating said incising laser beam is an excimer laser. 
     
     
       3. A system as recited in  claim 1  wherein said detector for modeling the distorted wavefront is a Hartmann-Shack sensor unit. 
     
     
       4. A system as recited in  claim 1  wherein the stromal surface is exposed using a blade to create a stromal flap, the stromal flap being movable to expose the surface for photoablation of tissue. 
     
     
       5. A system as recited in  claim 1  wherein the desired wavefront is a plane wavefront. 
     
     
       6. A method for controlling the superficial photoablation of tissue from an exposed stromal surface of an eye wherein the exposed surface introduces mechanically induced optical aberrations, said method comprising the steps of: 
       predetermining a desired wavefront for the eye;  
       generating a diagnostic laser beam;  
       using said diagnostic laser beam to identify an induced wavefront, said induced wavefront being characteristic of the mechanically induced optical aberrations;  
       altering said desired wavefront with said induced wavefront to create a rectified wavefront;  
       generating an incising laser beam to photoablate tissue from the exposed surface;  
       using a deformable mirror to direct said diagnostic laser beam through the exposed stromal surface to a focal spot on the retina of the eye;  
       detecting a distorted wavefront with light from said diagnostic beam reflected from the retina through the exposed stromal surface, said distorted wavefront having actual real-time characteristics of the cornea;  
       comparing said rectified wavefront with said distorted wavefront to create an error signal;  
       reconfiguring said deformable mirror in accordance with said error signal to maintain said focal spot on the retina; and  
       ceasing generation of said incising laser beam when said error signal is a null.  
     
     
       7. A method as recited in  claim 6  wherein said step for generating said incising laser beam is accomplished using an excimer laser. 
     
     
       8. A method as recited in  claim 6  wherein said step for identifying the induced wavefront is accomplish using a Hartmann-Shack sensor unit. 
     
     
       9. A method as recited in  claim 6  wherein the stromal surface is exposed using a blade to create a stromal flap, the stromal flap being movable to expose the surface for photoablation of tissue. 
     
     
       10. A method as recited in  claim 6  wherein the desired wavefront is a plane wavefront. 
     
     
       11. A method for controlling the superficial photoablation of tissue from an exposed stromal surface of an eye comprising the steps of: 
       detecting a distorted wavefront in the light of a diagnostic laser beam after reflection of said diagnostic laser beam from a focal spot on the retina of the eye;  
       extracting an induced wavefront from said distorted wavefront;  
       adding said induced wavefront to a desired wavefront to obtain a rectified wavefront;  
       comparing said rectified wavefront with said distorted wavefront to generate an error signal;  
       employing said error signal to control said focal spot of said diagnostic beam on the retina for subsequent reuse in said detecting step; and  
       monitoring said error signal as said distorted wavefront changes during photoablation of exposed stromal tissue to cease photoablation thereof when said error signal is at a null.  
     
     
       12. A method as recited in  claim 11  wherein said distorted wavefront includes optical characteristics of the uncorrected eye. 
     
     
       13. A method as recited in  claim 12  wherein said desired wavefront is a plane wavefront. 
     
     
       14. A method as recited in  claim 13  wherein the exposed surface introduces mechanically induced optical aberrations and said induced wavefront is characteristic of the mechanically induced optical aberrations. 
     
     
       15. A method as recited in  claim 14  wherein said employing step comprises the steps of: 
       using a deformable mirror to direct said diagnostic laser beam through the exposed stromal surface to said focal spot on the retina of the eye; and  
       reconfiguring said deformable mirror in accordance with said error signal to maintain said focal spot on the retina.  
     
     
       16. A method as recited in  claim 11  wherein said photoablation is accomplished using an excimer laser. 
     
     
       17. A method as recited in  claim 11  wherein said extracting step is accomplished using a Hartmann-Shack sensor unit. 
     
     
       18. A method as recited in  claim 11  wherein the stromal surface is exposed using a blade to create a stromal flap, the stromal flap being movable to expose the surface for photoablation of tissue. 
     
     
       19. A method as recited in  claim 18  wherein said blade is a microkeratome.